1. Field of the Invention
This invention pertains to an apparatus for cooling blown polymer films in a cryogenic flash chamber using a sprayed cryogenic fluid which vaporizes at an extremely low temperature for cooling many gauges and types of polymer films.
2. Description of the Prior Art
Many methods are presently known for cooling blown tubular polymer films. A recent example in the patented art is U.S. Pat. No. 3,576,929, which is directed to a method for cooling blown, thin gauge polymer films.
In conventional blown film manufacturing an air ring is the primary method of reducing the temperature of the polymer to the crystalline state from the molten state. The air ring may use refrigerated air or external ambient air to aid in heat transfer. Several other proprietary processes are available using an internal or external mandrel, water process, or an internal cooling device. The goal of all these devices is to produce a better, more uniform product at a higher production rate. Mechanical film properties improve with some of these processes.
In the present state of the art process, the molten polymer is extruded from an annular die. An "air" ring blows air on the material to reduce the temperature by convection and aids in bubble stability. The frost line is the area where the molten material crystallizes. The frost line should remain relatively close to the die to maintain a uniform process, i.e. on a 20 inch diameter bubble, the frost line should be approximately 18 to 24 inches above the die. Above the frost line the film cannot yet be collapsed until it has cooled down to about 125.degree. F. If the film is collapsed too soon, the pressure from the nip roll and lack of air between layers of film will cause the film to block, that is, stick to itself. To let the film get as cool as possible, the time period from when the material leaves the die to when it collapses should be as long as possible. Obviously, the slower the speed the longer the time period, but this also means slower production. By having the distance as great as possible between the die and nips, maximum production is possible with minimum degradation of properties.
The present art has two major deficiencies. The first deficiency is the slowness of the cooling process. This serves a limiting factor on the production rate of the extruder. The second major deficiency is that the present methods limit the types and gauges of polymer films which can be extruded and cooled. The apparatus and method of the present invention overcomes these deficiencies by spraying a rapidly vaporizing cryogenic fluid near the frost line to cool the blown film more quickly. It also utilizes the extremely cold vapors of the cryogenic fluid to feed an "air" ring, thereby commencing the crystallization of the molten polymer at a faster rate than presently possible. This, in turn, permits the cooling of a wider variety of polymer films. The addition of a conductions device will enable the most efficient heat transfer.